Catalyst formulations used in hydrocarbon conversion processes have been the subject of a very large number of studies. Patents and publications demonstrating that the addition of promoters to a base metal improves the quality of catalysts exist in large numbers. For paraffin dehydrogenation catalysts, catalysts comprising, in addition to a support, a noble metal from the platinum family and at least one additional metal M (United States patents U.S. Pat. No. 3,998,900 and U.S. Pat. No. 3,531,543) have been known for a long period. The acidity of the refractory inorganic support can lead to undesirable secondary reactions such as cracking and isomerisation. For this reason the oxide support is generally neutralised by adding at least one alkali or alkaline-earth element.
The metal phase is the hydro-dehydrogenating function which dehydrogenates the paraffins and hydrogenates coke precursors. However, platinum has a hydrogenolysing activity which deleteriously affects the yields of the desired olefins in the paraffin dehydrogenation process. This hydrogenolysing activity can be substantially reduced, and thus the selectivity of the catalyst can be increased, by adding an additional metal M. Further, adding this element M can also increase the hydrogenating properties of the platinum, encouraging hydrogenation of the coke precursors and thus increasing the stability of the catalyst.
Such elements are added in different forms such as mineral salts or organometallic compounds. The manner in which such modifying agents are introduced is not unimportant as it dictates the properties of the catalyst to a great extent. Thus a metal M is advantageously introduced using an organometallic compound of that metal M. Such a technique for introducing a metal M has been described in U.S. Pat. No. 3,531,543. Metal M is introduced in the form of at least one organometallic compound selected from the group formed by complexes, in particular carbonyl or polyketone complexes of metals M, and metal hydrocarbyls of metal M such as alkyls, cycloalkyls, aryls, metal alkylaryls and metal arylalkyls.
Introducing the additional element M in the form of an organometallic compound leads to more effective catalysts but necessitates the use of an organic solvent. The impregnating solvent described in United States patent U.S. Pat. No. 4,548,918 is selected from the group formed by oxygen-containing organic solvents containing 2 to 8 carbon atoms per molecule, paraffin, naphthene or aromatic hydrocarbons essentially containing 6 to 15 carbon atoms per molecule, and halogen-containing oxygen-containing organic compounds containing I to 15 carbon atoms per molecule. Such solvents can be used alone or mixed together.